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New Ways to Finish DMLS, MJF Parts

Eric Utley
By Eric Utley Technical Sales Engineer, Protolabs
An operator replenishes the powder supply in an SLS printer. (Photos supplied by Protolabs)

It seemed to happen in the blink of an eye. The 3D-printing industry has reached maturity. At this point, the use of 3D printing for rapid prototyping is ubiquitous, even among the smallest companies and hobbyists, and the desktop 3D-printing marketplace continues to boom.

Over the last decade, large corporations such as General Electric Co. and Hewlett-Packard Co. entered the industry with the intent to reliably produce end-use parts that are held to high engineering requirements. The number of industrial-quality 3D printers for sale and in service has grown tremendously in the last few years.

However, for all of their expense, these engineering marvels typically can’t produce parts that are in a fully finished state right out of the chamber. At a minimum, parts need to be cleaned of excess material or may require more advanced operations such as support removal, coloring, heat treatments, or advanced surface treatments.

While the number of parts printed per year grows, so does the demand for more automated processes downstream that reduce labor, while improving part quality and consistency. In recent years, new companies have surfaced offering solutions to these post-print problems; this additional level of complexity will bring the additive manufacturing (AM) industry into new levels of capabilities.

DMLS and MJF Parts and Texture

In the pursuit of enabling production-quantity 3D printing, the historical focus has been on improving printer throughput and material properties. Multi-jet fusion (MJF) and direct metal laser sintering (DMLS) printing technologies have achieved considerable success on this front. A single HP 5200 series MJF printer can print more than 1,000 copies of a small part (think the size of your finger) in a single overnight build. For DMLS, systems with multiple lasers are now the norm and significant advancements have been made to improve print speed.

Size is also becoming less of a barrier with DMLS systems, like the GE X Line, which not only print large, but can also produce higher quantities of midsize parts more economically. The material properties for both processes are excellent, with MJF producing plastic parts out of durable nylon, thermoplastic polyurethane TPU, and other common engineering thermoplastics. DMLS makes dense metal parts out of stainless steel, titanium, and aluminum.

However, for both technologies, the printed parts need extensive processing once the printing is done. MJF parts need to be broken out of unsintered plastic powder and blasted free of loose particles. Dyeing MJF parts black in post processing is an extremely common practice across the industry. DMLS parts need to be stress relieved, cut from supports, and receive extensive surface finishing to improve the surface quality.

Because these processes build a part layer-by-layer via powder sintering, the surface finish of the printed parts is somewhat coarse. As printed, MJF parts have a textured finish like blue jeans or unpainted drywall. DMLS parts have a coarse finish similar to cast metal, much like cast-iron cookware. Both can be improved with manual operations such as sanding, polishing, and machining, but this can add substantially to the cost and lead time, particularly if the finishing is done part-by-part in serial instead of within a bulk process.

There is a lot of market demand for smooth MJF and DMLS parts. The as-printed finish for MJF has reasonably good aesthetics and does not have very distinct layering, but the texture is hard to clean and gets dirty quite easily. For DMLS, the rough finish makes it difficult to use in myriad biology applications since it can damage cell walls or slow the flow of liquids and gases through channels.

Traditional DMLS finishing processes, as shown here, are time consuming and require skilled labor.

Smooth Technologies Make Smooth Parts

Two exciting technologies growing in popularity are vapor smoothing and dry electropolishing. DyeMansion North America Inc. and AMT Inc. offer specialized equipment that can leverage vapor smoothing, a chemical means to dramatically improve the surface finish of MJF and selective laser sintering (SLS) parts.

Meanwhile, a DLyte machine performs a dry electropolishing process that significantly smooths the surfaces of DMLS-printed parts. The common quality of all these processes is that they complement additive’s ability to do high-mix/low-volume production runs. The processes are robust enough to be performed on many different geometries without significant change in setup, and by processing parts in batch, they are able to keep up with the increasing throughput of production 3D printers.

Smoothing Parts with a Cloud

Vapor smoothing is an exciting development for plastic 3D printing, particularly for SLS, MJF, and other powder-sintering processes. The process puts parts in a vapor cloud and a solvent condenses onto the surface of the part, essentially melting it. The solvent causes the surface to become glossier and smoother. Several parts can be processed at one time, and because the process is driven by a gas cloud, it’s capable of finishing nearly all the surfaces of a complex geometry. Besides aesthetics, the finish seems to improve material elongation and makes the surface more resistant to liquids. The glossier finish is much easier to clean—it can be done with a cloth or wipe.

While the finish is greatly improved, it retains much of the inherent surface texture making the parts look like glossy leather. However, it’s still not economically viable to get plastic powder-sintered parts up to the glossy smooth finish that is possible with a polished injection-molding tool. Also, the process relies on a chemical reaction, so the chemistry of the base plastic must be compatible. Chemically resistant plastics, such as polypropylene, aren’t suitable for this process, whereas successful materials include nylon and TPU.

Smoothing Metal Parts with Ions

Dry electropolishing looks to be a way to economically improve the surface finish of metal printed parts by using spherical electrolyte beads to ionically remove part roughness. The powder-sintering manufacturing process creates a textured finish that persists even after grit blasting and spinning processes. If you increase abrasion to fully smooth the surface, you introduce the risk of compromising the geometry, particularly on sharp edges and corners. Traditionally, metal printed parts need to be either machined or polished by hand to reach premium finishes, but dry electropolishing can provide compelling results.

DLyte offers equipment that uses a dry, fine medium and essentially combines the principles of electropolishing with mechanical polishing to get the best of both worlds. The fine particles can reach into tight crevasses and improve the finish of all but the most inaccessible surfaces. These innovations are only possible because of the increasing scale of the additive industry. As the technology becomes even more widely adopted, there will be new opportunities for automated processes downstream from the print, but rest assured, the future is starting now.

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